“The Real Housewives of New York City” co-star Ramona Singer caused a stink on Friday night at Brasserie Cognac East on East 70th Street because a hostess had booked her daughter Avery’s high school graduation party for Sunday afternoon — even though the newly opened restaurant doesn’t serve lunch yet. Ramona stopped by Friday, opening night, to make sure everything was all set for the party of 10. When she learned Avery’s celebration would have to be postponed, Ramona burst into tears and began pleading with chef Florian Hugo. “She cried in front of the entire restaurant,” said a witness. Ramona confirmed the incident to Page Six — tears, tantrum and all. “This is my only child graduating high school,” she said. “I got so crazed, I said, ‘You have to help me.’ ” Managers offered to throw her party for free at Serafina, which is run by the same owners Vittorio Assaf and Fabio Granato — but Ramona was set on throwing the bash for Avery at Cognac East and declined their offer. “She insisted on it because the private party room is purple, and purple is her daughter’s favorite color,” our source said. After much begging by Singer, the restaurant agreed to open up just for Avery. We hear the party went off without a hitch, although Hugo had to bring in staff who hadn’t expected to be working.